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Chapter 21

Chapter 21. Oscillators. Objectives. Explain what occurs during an oscillation cycle. Identify various oscillators. Discuss and compare the Armstrong oscillator and the Hartley oscillator. Outline the operation of the crystal oscillator and the power oscillator. Basic Oscillators.

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Chapter 21

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  1. Chapter 21 Oscillators

  2. Objectives • Explain what occurs during an oscillation cycle. • Identify various oscillators. • Discuss and compare the Armstrong oscillator and the Hartley oscillator. • Outline the operation of the crystal oscillator and the power oscillator. Permission granted to reproduce for educational use only.

  3. Basic Oscillators • Made up of: • Wave-producing circuit • Amplifier • Feedback circuit • Cycle of voltage amplitude • Cycle passes in one period Permission granted to reproduce for educational use only.

  4. Sustaining Oscillation • Regenerative feedback, or positive feedback • Enough feedback voltage amplitude to replace energy lost Permission granted to reproduce for educational use only.

  5. Armstrong Oscillators Permission granted to reproduce for educational use only.

  6. Step One • Current flows from B- to B+ Permission granted to reproduce for educational use only.

  7. Step Two • First half-cycle of oscillation Permission granted to reproduce for educational use only.

  8. Hartley Oscillators Permission granted to reproduce for educational use only.

  9. Hartley Oscillators (Cont.) • Used in radio receivers and transmitters • More stable than Armstrong oscillators • Radio frequency choke (RFC) • Series fed oscillators • Shunt fed oscillators Permission granted to reproduce for educational use only.

  10. Shunt Fed Oscillators • No dc path through coil L1for emitter-collector current Permission granted to reproduce for educational use only.

  11. Review What is an oscillator? Electronic circuit that generates an ac signal at a desired frequency Permission granted to reproduce for educational use only.

  12. Review What is a complete set of events in a repeated series called? A cycle Permission granted to reproduce for educational use only.

  13. Review What differentiates a Hartley oscillator from other oscillators? The tapped coil Permission granted to reproduce for educational use only.

  14. Review What are the two types of Hartley oscillators? Series fed oscillators and shunt fed oscillators Permission granted to reproduce for educational use only.

  15. Other Oscillators • Colpitts oscillators • Crystal controlled oscillators • Power oscillators • Operation amplifier oscillators Permission granted to reproduce for educational use only.

  16. Colpitts Oscillators • Similar to Hartley oscillators • Split stator capacitor • Ganged capacitor Permission granted to reproduce for educational use only.

  17. Crystal Controlled Oscillators • Radio communications, broadcasting stations • Piezoelectric effect Permission granted to reproduce for educational use only.

  18. Crystal Controlled Oscillators (Cont.) • Crystals control transmitter frequency • Crystal ovens • Crystal added to feedback circuit Permission granted to reproduce for educational use only.

  19. Crystal Controlled Oscillators (Cont.) • Crystal used instead of inductor in the tank circuit of Colpitts oscillator • Pierce oscillators Permission granted to reproduce for educational use only.

  20. Additional Uses for Crystals • Two crystals producing two different frequencies for measuring temperature • Timing devices • Signal generators Permission granted to reproduce for educational use only.

  21. Power Oscillators • Collector load of each transistor is primary of transformer • Imbalance in conductivity starts oscillation Permission granted to reproduce for educational use only.

  22. Op Amp Oscillators • Crystal used in feedback circuit • NAND gate starts op amp output Permission granted to reproduce for educational use only.

  23. Wien Bridge Oscillators • Use positive and negative feedback • Balance of feedback in circuit crucial Permission granted to reproduce for educational use only.

  24. Existence of electromagnetic waves that travel at the speed of light Advances occurred prior to World War II Microwaves are transmitted out of radar dish in energy pulses When waves strike reflective material, some is reflected back Types of radar Pulsar radar Continuous-wave radar Doppler radar FM radar Some planes are “invisible” to radar Stealth bombers Stealth fighters Radio Detection and Ranging (Radar) Permission granted to reproduce for educational use only.

  25. Review What is the distortion in a crystal caused by a voltage applied to the surface of a crystal? Piezoelectric effect Permission granted to reproduce for educational use only.

  26. Review What is a Pierce oscillator? A Colpitts oscillator that uses a crystal in place of the inductor in the tank circuit Permission granted to reproduce for educational use only.

  27. For Discussion • Compare and contrast an electronic oscillator and a clock pendulum. • Explain the operation of a Hartley oscillator. • Why are crystal oscillators used in many commercial transmitters? Permission granted to reproduce for educational use only.

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